In the field of optics and optoelectronics, transparent polymers are increasingly being used since these materials offer advantages in terms of fracture resistance and weight saving. It is also possible in the field of precision optics to produce components with more complex three-dimensional geometry, such as objectives or lenses, in large numbers in a relatively simple manner.
Illustrative polymer materials which are currently being used in the field of optics are polymethyl methacrylate, polycarbonate, diethylene glycol bisallylcarbonate (trade name: CR39®), or else specific polythiourethane-based polymers of high refractive index.
One disadvantage of these polymer materials is the relatively low surface hardness and scratch resistance thereof.
A known approach to improving scratch resistance involves applying a surface coating via a sol-gel process. For example, tetraalkoxysilanes are used, which are hydrolysed under suitable conditions and then lead through condensation of the silanol groups formed by the hydrolysis to a three-dimensionally crosslinked silicate structure.
As explained above, a number of different polymers are now being used as substrates in the field of optical materials. Due to their particular chemical structure, these polymer substrates may differ significantly in terms of their surface properties, which can in turn have a significant influence on the adhesive strength of a coating composition on the substrate surface.
Coating compositions of this type ideally have good adhesive strength on different polymer substrate surfaces and high scratch resistance. The coating compositions also should ideally satisfy a number of further requirements. The coating should, inter alia, have a low tendency to crack formation under thermal stress and a high resistance to alkalis and/or acids. In implementing this profile of requirements, however, it should be taken into account that the abovementioned properties frequently oppose one another, and improvement in one of these properties can therefore be achieved only at the cost of another property.
In order to increase the flexibility of the silicate network and thus to reduce the tendency to crack formation under thermal stress, it is customary, for example, to use the tetraalkoxysilanes in combination with organoalkoxysilanes (i.e. silanes which, apart from alkoxy groups, also have one or more organic radicals bonded directly to silicon atom). The organic-inorganic network structure obtained as a result has higher flexibility and base stability, but at the cost of a low hardness compared to the purely inorganic silicate network.
U.S. Pat. No. 3,986,997 describes an aqueous composition which comprises colloidal SiO2 and an organotrialkoxysilane, for example methyltrimethoxysilane, and/or hydrolysis and/or condensation products of this organotrialkoxysilane.
In addition, it is also known from the prior art that tetraalkoxysilane or colloidal SiO2 and an organoalkoxysilane wherein the organic radical contains an epoxide group can be used together with a dicarboxylic acid or a dicarboxylic anhydride.
For instance, US 2001/0049023 A1 discloses a composition for the coating of a substrate based on an aqueous-organic solvent mixture which comprises (i) an organoalkoxysilane with epoxide functionality or the hydrolysis and/or condensation products thereof, (ii) a tetraalkoxysilane or the hydrolysis and/or condensation products thereof and (iii) a dicarboxylic acid or a dicarboxylic anhydride.
The use of tetraalkoxysilane or colloidal SiO2 and an organoalkoxysilane wherein the organic radical contains an epoxide group in combination with a dicarboxylic acid or a dicarboxylic anhydride is also discussed in U.S. Pat. Nos. 4,355,135 and 5,322,888.
WO 2008/087741 discloses a coating composition which comprises (A) a poly(methyl)glycidyl ether compound with an aliphatic R1 radical, (B) a silsesquioxane, (C) an alkoxy compound, (D) or organoalkoxy compound wherein the organic radical bonded to the silicon atom has a cationically polymerizable group, for example an epoxide group, and (E) a photopolymerization catalyst. The compositions described in WO 2008/087741 are free of solvents.